There is increasing awareness of the desirability of insect pest control by biological agents. Considerable effort in recent years has been devoted to the identification and exploitation of DNA viruses with large genomes, especially the baculoviruses. These viruses generally require extensive genetic manipulation to become effective insecticides, and the first such modified viruses are only now being evaluated.
In contrast, very little effort has been devoted to the study and use of small viruses with RNA genomes.
Four main groups of small RNA viruses have been isolated from insects. These include members of the Picornaviridae, the Nodaviridae, the Tetraviridae and the unclassified viruses. Descriptions of these groups can be found in the Atlas of Invertebrate Viruses (eds J. R. Adams and J. R. Bonami) (CRC Press, Boca Raton, 1991) and Viruses of Invertebrates (ed. E. Kurstak) (Marcel Dekker, New York, 1991). These disclosures relating to these viruses concern their pathology and biology, not their use in biological control.
Further information regarding small RNA viruses of insects an be found in P. D. Scotti et al (1981) “The biology and ecology of strains of an insect small RNA virus complex” Advances in Virus Research 26, 117-143. This review describes the insect picornaviruses cricket paralysis virus and Drosophila C virus (diameters estimated at 27-30 nm with one RNA component of 7.5-8.5 kb). N. F. Moore & T. W. Tinsley (1982) The small RNA viruses of insects. Brief review Archives of Virology 72, 229-245. This review included viruses of the following families:                Nodaviridae (diameter 29-30 nm, 2 RNA components totalling 4.5 kb)        Picornaviridae (diameter 27-30 nm, one RNA component of 7.5-8.5 kb)        Nudaurelia β family (now called Tetraviridae) (diameter around 35 mn, either one RNA of 5.5 kb or two totalling 8 kb)        
N. F. Moore, B. Reavy & L. A. King (1985) General characteristics, gene organisation and expression of small RNA viruses of insects. Journal of general Virology 66, 647-659. This reference defines small RNA viruses of insects as being those less than 40 nm in diameter. The review covers Picornaviridae, Nodaviridae and the Nudaurelia β family (now called Tetraviridae).
D. Hendry, V. Hodgson, R Clark and J Newman (1985) Small RNA viruses co-infecting the pine emperor moth (Nudaurela cytherea capensis). Journal of general Virology 66, 627-632 described viruses with mean diameters of 40 nm and 38 nm and one or two RNA components up to 5.5 kb in length.
Most recently, the term insect small RNA viruses has been used by one of the present inventors to cover three main recognised toxic groups: the Picornaviridae, the Tetraviridae and the Nodaviridae (P. Scotti & P. Christian (1994) The promises and potential problems of using small RNA insect viruses for insect control. Sains Malaysiana 23, 9-18).
These references illustrate a long standing usage of the term in this field of the term “small RNA virus” for viruses with certain characteristics as listed above. Another important characteristic of these virus groups is that they are not occluded, in contrast to many large viruses like the cytoplasmic polyhydrosis (RNA) viruses or the DNA baculoviruses, granulosis viruses and entomopox viruses. The term would also be applied to viruses not members of the three families listed above, as long as they satisfied the definition of being up to 40 nm in size. There are reports of such unclassified viruses (eg in Hendry et al. 1985). Moreover, the taxonomic status of some members of the Tetraviridae still requires clarification and it might even be possible for this family to be split, with HaSV and other members with two RNA components in their genome being separated from those with only one component, like the type member Nudaurelia β virus, which has not yet been sequenced. The above definition of “small RNA virus” would still cover all members of such virus families.